23 
In the upper sequence the results obtained from individual 
operators are arranged with respect to increasing mean errors, 
where the numbers above give the number of plates measured by 
each operator and the arrows indicate the spread of the mean 
errors for each operator. The range of the average per 
formance for the 34 operators is from ±1.1 to ±2.2 micron. 
The lower diagram presents the sequence of mean errors 
for operator #6, who has one of the smaller average mean errors 
(±1.37 micron), for a sequence of 78 plates covering an 
observation period of 15 months. Aside from showing the 
variations in performance, one may wish to discover a trend 
of improvement by a slight reduction of the average mean error 
with time. The presented curve is typical for the performance 
of the other operators, at their individual mean performance 
levels. 
Figure 10 is a histogram formed from the 645,872 com 
puted differences of the corresponding double measurements. 
Its adherence to a normal distribution is evidence that the 
measuring process as such appears to be sufficiently free of 
bias errors*, consequently, the average mean error of ±1.63 
microns can be accepted as an indication of the average pre 
cision in the measuring operation. The overall range for 
precision of measurement for individual plates is from ±0.76 
to ±2.4 micron. Both the instantaneous operator performance 
and slight variations in image quality, due to seeing con 
ditions, are sources affecting the differences in the pre 
cision of the measuring process. 
b. Analysis of the Accuracies Obtained in the Recon 
struction and Orientation of the Photogrammetric 
Bundle, Based on 930 Plates Reduced for the World 
Net Program Between July 1966 and June 1968 
The parameters considered necessary to establish the 
projective relationship between the updated star catalog 
data (compare Formula (3)) and the measured star image 
coordinates were given in Figure 6. 
The reference stars are stored on a tape, selected from the 
catalog of the. Smithsonian Astrophysical Observatory and 
limited to stars < 8th magnitude with a positional accuracy 
of < ±0"4. Corresponding weights are applied during the least 
squares solution for the single camera. The resultant mean 
error for the image coordinates as obtained from the